The invention relates to polycarbonate resins and their use in molding articles.
Aromatic polycarbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor; see for example U.S. Pat. No. 3,028,365. Although such resins have been found to be moldable under a broad range of molding conditions suitable for thermoplastic resins, only select polycarbonate resin compositions are useful for blow molding for the fabrication of such articles as water bottles. This is due to the unique requirements of a thermoplastic resin for blow molding operations; see for example the requirements for the branched polycarbonate resins described in U.S. Pat. Nos. 4,286,083 and 4,621,132.
The branched polycarbonate resins differ from most thermoplastic polymers used for molding in their melt rheology behavior. In particular, they exhibit high melt elasticity and high melt strength. Both of these are important in extrusion blow molding, particularly in fabrication by extrusion blow molding of relatively large articles. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. These advantageous properties of the branched resins are in large part a result of their non-Newtonian flow characteristics.
In the conventional blow molding operation, a tube of the heat-softened polycarbonate resin may be extruded vertically into a mold. The extrudate is then pressed unto the mold surfaces with a pressurized gas flow (usually air or inert gas), shaping the heat-softened resin. In practice, the desired physical characteristics of a blow moldable polycarbonate resin can be achieved preferably by a branched polycarbonate.
Currently, branched resins are typically synthesized from monomeric materials, the proper melt strength and viscosity being obtained by controlling the molecular weight and the level of branching agent. Thus, U.S. Reissue Pat. No. 27,682 describes the preparation of branched polycarbonates in a conventional interfacial reaction or from chloroformates. U.S. Pat. No. 4,415,725 describes a similar method which may employ a carbonyl halide such as phosgene (as in the interfacial procedure), a haloformate or a diaryl carbonate.
This is time consuming and expensive and any "off-specification" material produced is wasted, disadvantages which are not associated with the production of linear polycarbonate resins. A preferred method, disclosed in U.S. Pat. No. 5,021,521, is by reactive extrusion of a linear or branched polycarbonate with a branching agent. Copending, commonly owned application Ser. No. 08/653,166 describes solid state polymerization in the presence of a branching agent.